1. Field of the Invention
The invention relates to a turbocompressor disk having an asymmetrical circular groove for receiving hammer attachment blades. It is used in the aeronautical field.
2. Description of the Background
In a turbine engine different types of connections between the blades and the disks are known. GENERAL ELECTRIC FR-A-2 660 361 (corresponding to U.S. Pat. No. 5,067,876) describes one of the blade/disk connections, according to which the blade roots are helical. In addition, the angles on each side of the blade root are constantly different compared with the surface of the disk and are only equal towards the center of the root. Such a blade/disk connection is generally used for pinned blades.
ROLLS ROYCE FR-A-2 504 975 describes a hammer root blade, whose fitting requires no access recess in the disk.
GENERAL ELECTRIC U.S. Pat. No. 4,460,315 describes a hammer attachment blade root installed on a conical sleeve with equal angles of the bearing faces .beta. and .beta.', but having opposite gradients and with contact surfaces which are mutually radially displaced.
FR-A-2 697 051 (corresponding to U.S. Pat. No. 5,395,213) describes a compressor vane system with pinned cavities in which to a greater or lesser extent there is an increase in the material on the periphery of the disk in order to compensate the differences of the forces in the cavity.
Such a known blade/disk connection consists of axial attachments machined on the periphery of disk in order to permit the fitting of the blade roots.
Another conventional blade/disk connection design consists of a circular groove machined on the periphery of each disk. According to this design, the blades fitted in these grooves are of the "hammer attachment" type installed in the grooves Such a blade/disk connection design is illustrated in FIGS. 1A and 1B.
More specifically, FIG. 1A diagrammatically shows in a perspective view a disk according to the latter blade/disk connection design and having a symmetrical circular groove in which are inserted the hammer attachment blade roots. Thus, FIG. 1A shows the disk 1 with the circular groove 3, which has an upstream lip 5 and a downstream lip 7. Like all the blades mounted on the disk 1, the blade 2 has a blade root 4, whose shape is similar to a hammer, hence the term "hammer attachment blade".
FIG. 1B shows the same blade/disk connection as in FIG. 1A, but in a front view. It is possible to see in FIG. 1B the blade root 4 of the blade 2, the groove 3 of the disk 1, as well as the upstream lip 5 and downstream lip 7. FIG. 1B also shows that the blade root 4 has a shape which can be maintained in the groove 3, while being limited as regards movement by the lips 5 and 7 of the groove 3. These lips 5 and 7 have introduction slots 6 permitting the insertion of the blade roots in the groove 3. They also maintain the blade root 4 within the groove 3, no matter what the forces exerted on the blade 2.
FIG. 1B also shows that the groove 3 is symmetrical, lips 5 and 7 having curvatures, whose respective gradients P5 and P7 are inclined by the same angle .delta. with respect to a plane P perpendicular to the disk rotation axis. The rotation axis is not shown in FIG. 1B for simplification reasons. However, it is pointed out that it is substantially parallel to the base 2a of the blade 2.
The angles .delta. between the gradients P5 and P7 of the curvatures of the lips 5 and 7 and the plane P are equal, so that in this case the components R1 and R2 of the centrifugal force Fc induced by the blade 2 on the lips 5 and 7 of the groove 3 (which are also called contact forces on the lips 5 and 7) are equal and symmetrical with respect to the plane P. The resultant force R of these components R1 and R2 is consequently in the same sense and direction as the centrifugal force Fc and their points of origin are aligned on the radial axis of the disk.
Apart from the centrifugal force Fc, other forces stress the blade/disk connection and in particular stresses F1 and F2 transmitted by the sleeves 9 and 11, which interconnect the different compressor disks. These stresses F1 and F2 result from the aerodynamic axial forces induced by the blades and consequently differ between the individual blade stages.
Moreover, for design reasons, the upstream sleeves and downstream sleeves of the same disk generally have different geometries. Most frequently the upstream sleeve (sleeve 9 in FIG. 1B) is conical, whereas the downstream sleeve (sleeve 11 in FIG. 1B) is cylindrical. Therefore the upstream lip 5 and the downstream lip 7 of the circular groove 3 are not subject to the same stresses.
Moreover, with the compression ratio reached nowadays in turbine engines, very significant temperature differences are often encountered between the upstream and downstream sides of the compressor disks. Therefore very considerable differences in the service life periods of the upstream and downstream lips are very frequently encountered, which leads to supplementary maintenance costs.
It is finally obvious that the symmetry of the attachment introduces the risk of fitting the blade in the disk groove the wrong way round, i.e. placing the blade trailing edge in the upstream direction instead of the leading edge and vice versa.
FIGS. 2A and 2B show hammer attachment blades installed in the disk 1 of a conical stream, i.e. a stream inclined in the same way as would be encountered in a bypass turbofan engine with a very considerable bypass ratio. The conicity of the stream requires a significant clearance between the radial positions of the bearing faces of the lips 5 and 7, i.e. inner faces of the groove against which the blade root bears. This clearance influences the axial position of the resultant R of the components R1 and R2 of the centrifugal force on the lips 5 and 7. This resultant R is then misaligned with respect to the centrifugal force Fc, which means that the sense and direction of the faces R and Fc are identical, but their origins are not aligned on the radial axis of the disk.
Reference d in FIG. 2A represents the clearance between the resultant R and the centrifugal force Fc exerted in the center of gravity of the blade 2. Generally such a misalignment produces a parasitic torque on the blade 2.
FIG. 2A shows an example of a stream having a conicity such that the axis of the component R1 passes in the vicinity of the bearing face of the downstream lip 7.
Under axial stressing, the operational tolerances and the clearances between the blade root 4 and the groove 3 of the disk 1 enable the blade 2 to pivot about the center of rotation C located at the end of the downstream lip 7.
FIG. 2B shows the same blade/disk connection example as in FIG. 2A, but in the situation where the blade 2 is pivoting about the center of rotation C. FIG. 2B shows that the blade root 4 can escape from the disk groove 3.